FIG. 1 is a general exemplary block diagram showing an isolated DC/DC switching converter, which can be used for example for feeding a LED lighting source.
The main components of the exemplary diagram in FIG. 1 are the following:                1: mains supply,        2: Power Factor Correction (PFC) circuit,        3: output capacitor of PFC circuit,        4: power stage (primary side),        5: galvanic barrier (transformer),        6: power stage (secondary side),        7: light source (in itself not being part of the feeding device, but shown in dashed lines for completeness of illustration).        
The diagram in FIG. 1, which is of course wholly exemplary, provides in addition further elements, such as:                8: control circuitry (primary side),        9: control circuitry (secondary side),        10: auxiliary power stage (primary side),        11: auxiliary power stage (secondary side),        13: control board (primary side),        14: control input,        15: control board (secondary side),        17: auxiliary supply towards power stage (primary side) 4,        18: supply of control circuit 8 (primary side),        19: control line from circuit 8 towards power stage (primary side) 4,        20: supply of control circuit 9 (secondary side),        21: auxiliary supply towards power stage (secondary side) 6,        22: control line from circuit 9 towards power stage (secondary side) 6,        25: auxiliary supply from stage 10 towards control board (primary side) 13,        26: auxiliary supply from stage 11 towards control board (secondary side) 15, and        27: connection between control board (secondary side) 15 and power stage (secondary side) 6.        
The general layout shown in FIG. 1 is known in itself, which makes it unnecessary to provide a more detailed description both of the structure and of the operation thereof.
In this respect it will be moreover appreciated that the exemplary embodiment in FIG. 1 includes various elements (e.g. the elements denoted by references 13, 14, 15, 25, 26, 27 or others) the presence whereof is wholly optional and/or which are adapted to be replaced, in part or completely, with equivalent elements.
For what of interest here, in FIG. 1 it is possible to identify two main parts of the converter circuit, namely a primary side and a secondary side mutually separated by a galvanic barrier 5, being embodied for example by a transformer and being adapted to ensure safety for a user in case of an accidental contact with the load (light source 7).
In some embodiments, the control circuit can include, on primary side 8 and/or on secondary side 9, “smart” logical circuits, adapted to control the behaviour of the converter circuit as a function of prescribed operational parameters.
In some embodiments, this may involve having to “save” data related to the device operation, by storing them in a non volatile memory if a failure of the input power supply 1 takes place (for example in the case of a major or minor voltage drop or of a complete interruption).
The diagram in FIG. 1 shows the possibility of providing a control board (essentially elements 13 and 15) adapted to interact with the supply unit and, for simplicity of illustration (but by no way as a compulsory condition), the circuitry performing such a saving function can be considered as being included in, or anyway dependent from board 15.
FIG. 2 (wherein parts, elements and components identical or equivalent to those described with reference to FIG. 1 are denoted by the same reference numbers as in that Figure) shows the possibility of providing, on the secondary side, an energy storage element 12, the function whereof is to supply the save circuitry during saving the data which must be stored.
The energy storage element 12 can be supplied by a charging circuit 23, which derives from secondary side 6 of the power stage the energy which must be stored in the energy storage element 12. In various embodiments, element 12 can be a capacitor, which stores the energy that can be used as a power supply when a failure from the power supply 1 from the mains takes place, i.e. when the input power supply from the mains is no longer available or sufficient to ensure the regular operation of the converter device.
In various embodiments, in the storage element 12 a sufficient amount of energy can be stored as to support the operation of the save circuitry (for example located in board 15) for the whole time interval needed to save the parameters, in presence of a failure of primary input power supply 1. This time interval depends in turn on the amount of data to be saved, also considering the power absorbed by the circuit during such operations.
The energy that can be stored in a capacitor can be expressed as ½·CV2 (wherein C is the capacity and V is the charge voltage). In order to keep the output voltage at a suitable low value for a time interval sufficient to save the data, a rather high capacity C is required, which involves the use of a capacitor with large size and high cost.
For example, if a control system according to the DALI standard (Digital Addressable Lighting Interface) is used, in case of a drop of the input voltage approximately fifty parameters are stored for each channel, i.e. for each independently controlled LED string. The time interval needed for the data save operation can be 200 ms for each channel, so that on the whole, in the presence of N independently controlled LED strings, storage element 12 must be able to store a sufficient amount of energy to support the operation of the data save circuitry for a time interval of approximately N×200 ms.
In various embodiments, stage 2 (see FIG. 1), which is used for the power factor control (PFC) of input current and to stabilize the output voltage, can be implemented with a boost converter having a supply voltage higher than the maximum peak value of the rated input voltage. For example, in the case of a 200 V alternated input, the PFC stage output can be 400 V DC, applied across capacitor 3.
When the input power supply fails, the voltage on capacitor 3 starts dropping and the energy stored in the capacitor itself is absorbed by the main power stage 4, 6 (and therefore by load 7) and by auxiliary power stage 10, 11.
For this very reason, in various embodiments the need can be felt of providing energy storage element 12, adapted to supply the circuit for the remaining time needed to complete the save operations. Such an energy storage element can be arranged proximate to the smart circuit to be fed.
Generally speaking, the generation of auxiliary voltages within a feeding device in the lighting sector is a topic which has been extensively dealt with, also in patent literature. In this respect it is possible to refer, for example, to document WO-A-2008/128575, to document WO-A-2008/138391 and to European Patent Application EP 11159153.3.